1. Field
Implementations of the present invention relate generally to the field of photodiodes and more particularly to an avalanche photodiode (i.e., APD) with a reduced operating voltage exhibiting high speed over a large range of current densities and, under applicable conditions, reduced after-pulsing.
2. Brief Description of an Illustrative Environment and Related Art
Avalanche photodiodes are incorporated in many high performance optical communications, imaging and sensing applications because they enable high signal to noise ratio and high-speed operation. Arrays comprising pluralities of APD elements are indicated in imaging applications. Uniformity of operating voltage among the APD elements within an APD array is an important parameter in achieving accurate imaging information and wide dynamic range. Typically, an individual InGaAs/InP APD operates at between 20 and 70 volts, with an array of such APDs exhibiting operating-voltage non-uniformities among the constituent APDs of about 3 to 7 volts. Such a large range of non-uniformity in operating voltage has heretofore been regarded as an inherent characteristic of InGaAs/InP APD arrays requiring mitigation by relatively complex bias circuitry and reducing the overall dynamic range of the imager.
In Geiger-mode applications, after-pulsing results from charge carriers (i.e., electrons and holes) being trapped and later released from hetero-junction boundaries in the APD structure. A generally undesirable phenomenon, after-pulsing yields larger false photon counts and decreases the overall photon-detection probability of the system in which the after-pulsing APD is included. After-pulsing is especially present in InGaAs-InP APDs in which charge carriers generated in the InGaAs absorber must surmount a potential barrier before being injected into the multiplication region.
Homo-junction, silicon-based APDs have been applied to imaging-sensor array and Geiger-mode applications and, although the homo-junctions alleviate the after-pulsing phenomenon associated with hetero-junctions, silicon is not well suited for optical detection of wavelengths outside the range of about 0.35 to 1.0 microns. Consequently, silicon-based APDs are not sensitive to electromagnetic wavelengths in the 1.2 to 1.7 micron range, a range that is of importance to many applications.
FIG. A is a generalized representation of the structure of a typical, extant avalanche photodiode. The architecture of the APD in FIG. A is grown upon a substrate and includes a charge-carrier multiplication region and an intrinsic optical absorption region having first and second opposite sides. The charge-carrier multiplication region includes p-doped and n-doped sub-regions adjacently bound through a homo-junction. The charge-carrier multiplication region is oriented such that the p-doped sub-region forms a hetero-junction with the first side of the intrinsic absorber and the n-doped sub-region is adjacent the substrate and constitutes the cathode of the APD. In existing APDs, the absorption region is fabricated from an intrinsic first material having a first conduction band potential and the charge-carrier multiplication region is fabricated from a second material having a second conduction band potential that is higher than the first conduction band potential. Adjacent the second side of the intrinsic absorption region is a p-doped region constituting the anode of the APD. In order to introduce a reverse bias voltage across the APD, the anode and cathode of the APD are placed in electrically conductive engagement with, respectively, the cathode and anode of an external energy source.
Under reverse bias, the p-doped regions assume a negative charge and the n-doped region assumes a positive charge. With the introduction of a photon of sufficient energy hv into the intrinsic absorption region, a charge-carrier pair including a positively-charged hole and a negatively-charged electron is generated by the movement of an electron from the valence band into the conduction band as is well known by those possessing even a rudimentary understanding of physics or chemistry. The electric field induced across the APD by the application of the reverse bias causes the hole to move toward the negatively charged p-doped APD anode. Conversely, the freed electron is accelerated toward the charge-carrier multiplication region. However, the higher conduction band potential of the second material from which the multiplication region is fabricated presents a potential barrier to approaching electrons at the homo-junction. Moreover, the negatively charged p-doped sub-region in the multiplication region presents a second obstacle. Those electrons that acquire sufficient kinetic in the absorption region to overcome both obstacles are introduced into the charge-carrier multiplication region where they are greatly accelerated by a relatively high-magnitude localized electric field attributable to the juxtaposed p-doped and n-doped sub-regions. Each electron of a selected plurality of the electrons entering the multiplication is sufficiently accelerated to collide with and “knock out” still additional electrons from the crystalline lattice of the multiplication region (i.e., to undergo impact ionization). A secondary electron freed by impact ionization may itself undergo impact ionization but, on average, at least contributes to the overall current.
Depending on devise parameters as well as the magnitude of the reverse bias voltage VRB, a portion of the “primary” electrons freed by photon absorption in the absorption region will lack sufficient energy to overcome the obstacles presented by the mismatched conduction band potentials of the first and second materials and the negatively charged p-doped sub-region of the multiplication region. Over time, these less energetic electrons accumulate near the hetero-junction and eventually themselves contribute yet an additional obstacle to the movement of primary electrons into the multiplication region by increasing the magnitude of the negative charge confronting an approaching primary electron. Accordingly, the applied reverse bias has to be substantial enough to ensure that primary electrons overcome the potential barrier at the hetero-junction. Moreover, conventional hetero-junction APDs suffer from low speed and after-pulsing due to the charge-carrier “pile-up” at the energy barriers of the hetero-junctions.
Accordingly, a need exists for an avalanche photodiode that operates at relatively low voltages and that exhibits minimal after-pulsing and very high speed over a broad range of current densities.